47 



to foreign bodies by a bony plug which passes through the substance of 

 the lower or left valve. In Placunanomia the animal attaches itself also 

 by a bony plug, but the orifice through which it passes is protected by a 

 rude divaricate tooth, or rather double-tooth, which appears in a more 

 symmetrical form in Placuna, where the orifice and plug of adhesion are 

 wanting. Ostrea has neither plug nor hinge. 



Anomia. Placunanomta. Placuna. Ostrea. 



Genus 1. ANOMIA, Linnaeus. 



Animal ; body massive, shaped like the shell, with the mantle 

 freely open, having pendent margins bearing a double fringe of 

 short cirrhi ; no siphonal tabes ; foot very small, often nearly 

 obsolete ; adductor muscle divided into three portions, the long- 

 est of which attached to a shelly plug passes through an orifice 

 in the lower valve; branchial leaflets doubled on themselves; 

 mouth surrounded by membranous borders and two pairs of 

 long labial tentacles ; sexes distinct. 

 Shell ; generally orbicular, inequivalve, irregular, affixed, some- 

 times rather solid, mostly very thin, hyaline • lower valve flat, 

 orbicularly perforated and notched near the hinge, perforation 

 filled with a subcalcareous plug of adhesion ; upper valve con- 

 vex, sauamately laminated, striated or ribbed, with three sub- 

 central muscidar impressions ; hinge toothless, with the ligament 

 short, attached to a callosity. 



The Anomia are chiefly remarkable for the manner in which they affix 

 themselves to foreign bodies. The adductor muscle divides into three 

 portions, and the central portion passes out through an orifice in the sub- 

 stance of the shell for the purpose of anchorage, and secretes a bony plug, 

 which serves as an operculum. As in all mollusks of parasitic habits, the 

 shell varies exceedingly in the same species, becoming more or less shaped 

 to the irregularities of its place of attachment. If growing upon a ribbed 

 surface, the shell assumes a ribbed structure; but the ribs formed under 

 these circumstances are not characterized by the definition which belongs 

 to a species that has a natural ribbing of its own. 



There is, perhaps, more difficulty in discriminating between the different 

 states of a species incident to age and habitat. The well-known A. ep/iip- 

 pium of the European Seas has been described twenty times over. Even 

 of the nine recent Anomics described by Lamarck, eight are supposed to 



